Generally, gas engines are internal combustion engines that use gas as fuel, which are advantageous in that fuel expense can be reduced compared to that of gasoline fuel, thermal efficiency is comparatively high thanks to combustion efficiency that is close to that of complete combustion, and the pollution rate from exhaust gas is low.
Meanwhile, to enhance the efficiency and output of such a gas engine, a precombustion chamber structure which ignites fuel gas before supplying it into a main combustion chamber is provided in a cylinder head of the gas engine
FIG. 1 is a sectional view of a conventional precombustion chamber structure for a gas engine.
The conventional precombustion chamber structure includes a body 10 which has formed therein a precombustion chamber 30 for precombustion of fuel gas, and a spark plug 20 which is installed in the body 10 to ignite fuel gas injected into the precombustion chamber 30.
The body 10 includes a main body 11, a water jacket 12 and a precombustion chamber tip 13. A gas supply passage 14 through which fuel gas is supplied into the precombustion chamber 30 is formed in the water jacket 12. A check valve 15 is provided in the main body 11 so as to control a flow path defined by the gas supply passage 14 and prevent combustion gas from flowing backwards.
The water jacket 12 is coupled to a lower end of the main body 11 in such a way that the water jacket 12 encloses the spark plug 20. A coolant circulation passage 12a for circulation of coolant is formed in the water jacket 12 to prevent the spark plug 20 from being overheated.
The precombustion chamber tip 13 is coupled to a lower end of the water jacket 12. The precombustion chamber 30 is formed by the coupling between the precombustion chamber tip 13 and the water jacket 12. Communicating with the main combustion chamber, a plurality of injection holes 13a are formed in a lower end of the precombustion chamber tip 13 so that ignited fuel gas flows into the main combustion chamber through the injection holes 13a. 
In the drawings, reference numeral 40 denotes a gasket.
The conventional precombustion chamber structure has the following problems.
First, it is not easy to disassemble the precombustion chamber tip so that maintenance, repair or inspection of the interior of the precombustion chamber is difficult.
Second, because the gas injection passage connected to the precombustion chamber is short, combustible gas easily flows backwards towards the check valve. Such backflow gas causes a valve sticking phenomenon, whereby the check valve is easily damaged.
Third, the sealing structure is not reliable, thereby increasing the risk of gas leakage.